1. Technical Field
The present invention is directed to a system for correcting optical aberration created by conformal windows. More specifically, the present invention is directed to a system for correcting optical aberration utilizing static and dynamic aberration correction means.
2. Discussion
Airborne optical sensors must be isolated from the environment to function properly and to preserve the aerodynamic effect of the aircraft design. This isolation is usually accomplished by placing the sensor behind a window. Current electro-optical sensors on high performance aircraft are primarily limited to pod mounted configurations which are a primary source of aircraft drag and radar cross-section signature.
The window must be designed to provide the proper field of regard for the sensor. What is meant by the field of regard is the complete set of ordered pairs of values representing azimuth and elevation viewing angles through which the sensor can be pointed or gimbaled. In context to this specification, this is in contradiction to the field of view which corresponds to the locus of points which the sensor can instantaneously observe given a particular orientation within the field of regard. Typically the field of view of an optical sensor is significantly smaller than its field of regard.
Aircraft window design for isolating optical sensors is driven by two considerations: maintaining the aerodynamic efficiency of the overall aircraft design and the need to render the field of regard of the sensor as free of optical distortion and aberration as is practicable.
Conformal windows, which are windows having contours matching those of the surrounding surfaces of the aircraft in the context of the present application, offer the most favorable aerodynamic shape for maintaining the overall efficiency of the aircraft design. However, conformal windows create considerable optical aberration which varies greatly as a sensor is gimbaled or pointed through the field of regard.
As a consequence to the considerable optical aberration created by the conformal windows and the difficulty of correcting such widely varying amounts of aberration, designers of on-board sensor systems typically prefer to use flat or spherical windows to protect sensors from airborne environments. Although the use of non-conformal windows benefits the sensor designers, the aircraft suffers from increased air resistance as a result of this design choice. The principal type of optical aberrations associated with conformal windows are coma and astigmatism. What is meant by coma in the context of the present application is the variation of magnification as a function of the aperture. Also in the context of the present application, astigmatism is the difference in focus location for fans of rays in the sagittal and tangential planes.
In previously known systems, the problem of correcting coma and astigmatism due to the use of conformal windows has not been resolved by optical means. A device for generating optical aberration has been previously disclosed, e.g. see Aberration Generator by R. A. Buchroeder and R. Brian Hooker, Journal of Applied Optics (1975), however this device provides limited amounts of optical aberration when compared to the amount of optical aberration required in the context of the present invention. For this reason, the aberration generator disclosed in the above mentioned reference is inadequate for the present application. As above mentioned, absent an acceptable optical solution to aberration correction, designers are forced to use flat or spherical windows as mentioned above. Because these window designs reduce aerodynamic efficiency, smaller windows may be used to limit aerodynamic degradation. As a result, the sensor field of regard is limited.
U.S. Pat. No. 5,526,181, assigned to the Assignee of the present invention, the specification and drawings of which are herein expressly incorporated by reference, discloses a device compensating for the optical aberration created by a conformal aerodynamic window having a generally cylindrical shape. A one-dimensional corrector plate provides a varying amount of coma. The corrector plate has aspherical deviations in one direction only and is capable of being displaced in a direction perpendicular to an optical axis of the system. The corrector plate further has an axis of bilateral symmetry while lacking an axis of rotational symmetry. While this system works adequately for its intended purpose, the corrector plate is difficult to manufacture and the corrector plate needs to be decentered which decentering motion is the hardest to implement in the system. The invention also depends on the shape of the window being cylindrical, which is typically not the most desired conformed shape.
Future sensors must be internally mounted and look through windows that conform to the aircraft shape in order to achieve desired increases in aerodynamics and radar cross section performance. These goals must be accomplished while retaining optical performance without a huge increase in system complexity. The system should be simple, easy to build and install, and cost effective.